Hi all: How can I compute the Return Loss coupled into source fiber due to back reflection in a system?

For example: I would like to know how much light is reflected back from the Image surface with AR coating back into my source fiber as indicated by the BLUE arrow. 

One way would be to make the Image Surface material == MIRROR, but how can I determine how much light is coupled back into Source fiber originating from the Image Surface AR coating?

Example design file can be downloaded from here.

Cheers!